Sulphur

Sulphur

Most of the sulphur used in the industries is derived from the native mineral, which is found in many places, but usually in volcanic regions. It is always impure, being mixed with gypsum, aragonite, clay, or other matter, in the interstices of which the sulphur is deposited. The formation of sulphur beds may have occurred by the reaction of gases, such as hydrogen sulphide and sulphur dioxide, with each other or with oxygen; or by the decomposition of metallic sulphides through the agency of heat; or by the reduction of sulphates, especially of calcium sulphate. The first is probably the most frequent mode of deposition, and may be observed at the present time in many volcanic districts where hydrogen sulphide and sulphur dioxide are escaping. The reactions are the following:- These gases are always present where volcanic action is in progress. The reduction of suIphates has probably caused the formation of some stratified deposits. By far the largest part of the world's supply of sulphur comes from Sicily, but some is obtained in Japan, Italy, Greece, and in the United States, particularly near Humboldt, Nevada, at Clear Lake, California, and in Louisiana. But the price of the foreign product is too low to allow profitable working of the deposits in this country. In Sicily it is disseminated through the matrix, sometimes in considerable masses of nearly pure sulphur, but usually in fine seams or grains. The methods of obtaining it are very crude andwasteful. 'The mines are for the most part open pits, ranging from 200 to 500 feet in depth, and the ore is carried to the surface in baskets or sacks by laborers, who ascend by inclined paths on the walls of the pit. In a few of the better mines, however, hoisting machinery is now used, but only after overcoming the determined opposition of the laborers. The ore is generally refined in a very simple manner, the process being carried on in kilns called "calceroni." As usually constructed, these are shallow pits, about 30 feet in diameter, with walls about 10 feet high, made tight with mortar. They are generally built on a hill-side, and the sloping bottom is beaten smooth. The ore is arranged in the calcerone so as to leave a few vertical draught holes from top to bottom of the heap, which is fired by dropping burning brush or straw into these openings. The sulphur, forming from 25 to 40 per cent of the ore, burns freely, and when the heap is well on fire, the draught holes are closed, the calcerone covered with spent ore, and the whole left for several days. The heat given out by the burning of part of the sulphur is sufficient to melt the remainder from the gangue, and it collects in a pool near a tap-hole, made in the wall at the lowest point. At intervals of a few hours, the melted sulphur is drawn off into moulds. If the temperature rises above 180 C., there is a large formation of plastic sulphur, which will not flow from the tap-hole. The time necessary to burn out a calcerone varies from 35 to 80 days, according to its size, the weather, and the nature of the impurities; e.g. much gypsum retards the process owing to the water it contains. Usually from a quarter to a third of the sulphur is lost as sulphur dioxide during the burning. As this causes much damage to' vegetation in the vicinity, the burning of calceroni is prohibited during the spring and summer months. It has been proposed to separate sulphur by heating with hot air, with steam under pressure or with superheated steam. But this is unprofitable on account of cost of fuel in those regions where sulphur occurs. It may be separated by a solvent, such as carbon disulphide, which may be recovered afterwards, but this necessitates an expensive plant. But treatment with a solution boiling above the melting point of sulphur has proved successful for some ores. The ore is placed in an iron basket or crate and suspended in a boiling solution of calcium chloride,* which boils at 125 C. Since sulphur fuses at 115 to 120 C., it melts and flows away from the matrix of stones, etc.; passing through the basket meshes, it falls to the bottom of the tank, and is drawn off and cast in moulds. After the sulphur is melted out, the basket of hot stones is lowered into a tank of water, which is thus heated by the hot stones, while it removes the calcium chloride from them. This warm water is then used to replace that lost by evaporation from the boiling calcium chloride solution. This process causes no loss of sulphur as sulphur dioxide, and no nuisance is created, while a fairly pure product is obtained. The calcium chloride used is a waste product of the ammonia soda industry. In this country, extraction with superheated steam has been tried and yields an excellent quality of sulphur, without any formation of sulphur dioxide. But the cost of fuel in the West is an obstacle to further development. In Louisiana, it is proposed to force steam, under pressure, through driven wells or tubes, into the sulphur deposit. The sulphur, having been liquefied by the heat, is forced to the surface by the steam pressure, through a small pipe inside of the steam pipe. The industrial success of the process is as yet somewhat problematical. Iron pyrites (FeS2), when heated in a closed retort, yields one atom of sulphur per molecule of sulphide; and has been used as a source of sulphur, but the process is not now employed. A small portion of the sulphur of commerce is that known as recovered sulphur. This is chiefly obtained from the calcium sulphide waste of the Leblanc soda process, although a small quantity comes from the residues from the purification of illuminating gas by means of moist iron oxide. When iron oxide is used to purify gas, the following reactions take place: -

Fe203 . 3H20 +3H2S = 2 FeS + S + 6 H20.

On exposure to the air, this moist ferrous sulphide is oxidized thus:

2 FeS +3H20 +3 0 = Fe2O3 . 3H2O +2 S.

Hence the iron oxide is "revivified" and may be used again, and on exposure to the air, oxidation of more ferrous sulphide takes repeating this operation a number of times, there is sufficient free sulphur to be profitably distilled by heating in a retort. But the quantity of recovered sulphur is very small in comparison with the total annual demand, and there seems but little prospect of any great advance in the industry. With the exception of recovered sulphur, the above processes yield a crude impure product, which, however, is good enough for a large number of manufacturing operations. But for some purposes a further purification is necessary. This is generally done by distillation in a cast-iron retort, or in Dejardin's apparatus (Fig. 20). The crude sulphur is melted in the vessel (C), heated by the waste heat of the fire, and is then run into the retort (B), heated directly by the fire. The vapors pass into the receiving chamber (E), which is usually made of brick. If the temperature of the chamber (E) is not allowed to rise above 110° C., the vapors condense at once to a fine powder, which is sold under the name of "flowers of sulphur." If the temperature of (E) rises much above 110° C., the vapors condense as a liquid, which is drawn off into moulds, forming the "roll brimstone" of commerce. The chief uses of crude sulphur are: for making sulphuric acid; as a germicide in combating Phylloxera, a disease of the grape (this disposes of nearly one quarter of the yearly production); and in making ultramarine and carbon disulphide. The principal uses of refined sulphur are: for making gunpowder and matches, and for vulcanizing rubber. Sulphur melts at 115°_120° C. ; it is a very poor conductor of heat and electricity, and dissolves easily in carbon disulphide, less readily in chloroform, benzol, turpentine, and other oils. Its specific gravity is 1.98-2.04. Sicily, owing to its favorable situation as a shipping point, the abundance of cheap labor, and the richness of its deposits will probably continue to supply the major part of the sulphur consumed. But Japanese sulphur has become a considerable competitor with the Sicilian, and a deposit recently opened in one of the New Hebrides islands (Tanna) gives promise of future importance.